Lamp electrode



Oct. 2, 1956 E. c. MARTT LAMP ELECTRODE Filed July 12, 1954 Inverfizov:Ernes t C. Mav t 't,

His A irt vnegy United tatcs ?att1t ZLAIWP ELECTRODE Ernest C. Martt,Chagrin Falls, Ohio, assignor to General Electric Company, a corporationof New York Application July 12, 1954, Serial No. 442,605

3 Claims. .(Cl. 313-344) .Thc present invention relates in general togaseous elec- .tric discharge devices of the high-pressure type and moreparticularly to an electrode construction for such devices.

High-pressure mercury vapor electric discharge lamps in which the arcdischarge is constricted into a narrow discharge of high intensity bythe high pressure of the mercury vapor atmosphere during operation ofthe lamp comprise an arc tube in the form of a sealed tubular vitreousenvelope of quartz or hard glass containing a measured andlimited amountof mercury which is usually all evaporated during operation of the lamp:to produce .a high-pressure, unsaturated mercury vapor atmosphere. Theare tubes for such lamps also contain a starting gas, a pair ofcooperating, main discharge supporting electrodes sealed into oppositeends of the tubes and an auxiliary starting electrode mounted near oneof the main electrodes.

A form of .main electrode of the self-heating type in extensive use atthe present time in such lamps comprises a straight refractory metalwire extending longitudinally 'into the arc tube from an end thereof, ahelically wound wire coil of refractory metal slipped over and attached.to the straight wire with the inner .end or tip .of the straight wireextending beyond the corresponding end of the wire coil. A sliver ofthermion'ically active metal, such as thorium, 'is inserted between thestraight wire and the wire coil and is covered "by the Wire .co'i'l.These three elements are welded together to constitute :the elect-rode.The operation of the lamp, in brief, is as follows: On the applicationof suitable potential, a glow discharge starts between the startingelectrode and the adjacent .main electrode. 'The ionization spreadsthroughout the arc .tubeand very quickly a glow discharge starts betweenthe main electrodes at the ends of the tube and covers the mainelectrodes. The main electrodes heatup under the bombardment elfects ofthe glow discharge and, under the rise in temperature of the electrodes.the thorium .gives off a copious flow of electrons which'initiatesan.;arc

discharge between the main electrodes. The usual cathode spot at eachend of the arc discharge occupies a restricted portion of the wire coilwhen the arc is first formed, but, .as the mercury pressure builds upinthe arc tube under the heat ofthe arc discharge and the dischargebecomes constricted by the high pressure, the cathode spot concentra'teson the tip o'fthe straightwire where it remains during stable operation'of the lamp.

The principal :object of the present invention is do provide 'a maindischarge supporting electrode which fa- -cilitates starting andimproves the maintenance of :light output of a high-pressure, aredischarge lamp. Other objects and advan'tages of the invention willappear :trom the following detailed description of a species ithercof,from the accompanying drawing, Jandfrom :the appended claims.

feature of :the invention is a closely wound helical coil of ifinerefractory .metal wvire, hereinafter designated :as ithe outer .coil,:slipped over .and attached to the usual wire coil, hereinafterdesignated as the inner -coilof the electrode in such manner that thetwo coils are co axial. The outer coil is welded at its center portiononly to the corresponding portion of the inner wire coil and is of finerwire so as to have a lower thermal inertia or heat capacity than thewire constituting the inner coil.

The turns of the portion of the outer coil between its welded centerportion and the tip of the straight center wire of the electrode areradially spaced from the .corresponding turns of the coaxial inner .coilto minimize heat transfer from the said turns of the outer coil to theinner coil. The outer coil terminates short of the tip of the electrodeand the inner coil is no closer longitudinally to the electrode tip thanthe outer coil.

Preferably, the turns of the outer coil extend beyond the correspondingturns of the inner coil in the direction of the aforesaid tip, but thisis not essential and good results are obtained even when said coils arecoextensive in the direction of the electrode tip, provided, of course,that the turns of the outer coil are radially spaced from the inner coilexcept at .the welded-together center portions of the coils.

I have demonstrated that, in an electrode of this struc- ,ture, when theglow discharge starts between the main electrodes of the lamp it quicklychanges to an arc discharge with the cathode spot occupying the turns ofthe fine w ire outer coil. Usually the cathode spot occupies the turnsof the outer coil nearest the tip of the electrode. These turns of theouter coil on which the cathode spot concentrates, due to their lowthermal inertia or heat capacity, attain a highly electron-emissive,main arc discharge supporting temperature under the influence of theglow discharge between the main electrodes in a shorter time than anypart of the prior electrodes not equipped with the fine 'wire coil ofthe present invention and, as a result of the higher electron emissivityof this part of the electrode, the cathode spot concentrates thereonduring starting. With the build-up of vaporpressure by ,the heat of theare, the cathode spot transfers to the tip of the electrode. Thus, thefine wire coil facilitates markedly the starting and stabilizing of themain arc discharge in the arc tube by heating up to an are dischargesupporting temperature quickly.

171133 5 also .demonstated that the outer line wire coil improves themaintenance of light output of a lamp, including the arc tube, 'byproviding additional protection for the thorium sliver to reduce lightabsorbing desposits of sputtered electrode material on the end walls ofthe .arc tube.

In practicing my invention an ,outer coil is added .to each of thestandard electrodes .used commercially here- .tofore in lampscf .agivenwattage. In other words, the only change necessary in the manufacture ofprior lamps of ,theabove type inorder to attain the advantages of theinvention .is the addition of an outer coil to each of the mainelectrodes.

The addition .ofan outer coil ,to each of the electrodes, .of course,adds to the mass of the electrodes and to its heat dissipating surface.In the absence of .the novel :structuralieatures of the electrode ofthis invention, such an addition would lengthen the starting ti ySlowing the rate at which the electrode attains a highly electron-.emissive, arcdischarge supporting temperature and would .also result:in a disadvantageous lowering of the electrode temperature duringoperation of thelamp.

I have demonstrated, however, that contrary .to what might be expected,the addition of an outer coil to .each of .thema'in .electrodes in .themanner and of the kind .descrihed hereinresults Iin lamps which startfaster, attain stable operation quicker and have a higher maintenance.of light output .t'hanidenticaHamps not equipped with the electrodeouter coils of the present invention. j ...In. the drawing accompanyingand forming part of this specification an embodiment of the invention isshown in which:

Fig. 1 is an elevational view of an arc tube having mounted within itsends main discharge supporting electrodes embodying the invention;

Fig. 2 is a similar view, partly in section and on an enlarged scale, ofone end of the arc tube shown in Fig. 1 and illustrating the inner partsof the electrode mounted within this end;

. Fig. 3 is a plan view of the inner end of the electrodes shown inFigs. 1 and 2.

The are tube shown in Fig. 1 of the drawing comprises a lighttransmitting tubular quartz envelope 1 having its ends closed byhermetic seals 2 and 3 of the flat pressed type. Cooperating maindischarge supporting electrodes 4 and 5 are mounted within opposite endsof the envelope 1. An auxiliary starting electrode 6 having a fine wireloop 6' in the path of the main discharge to minimize electrolysis ofthe quartz at the seal 2, as disclosed and claimed in the St. Louis etal. Patent No. 2,660,692, dated November 24, 1953, and assigned to theassignee of this application, is mounted in one end of the envelopeclosely adjacent the main electrode 5. A quantity of mercury, indicatedat 7, is contained within the envelope 1 which also contains a startinggas, such as argon, at a pressure of about 2 centimeters.

The quantity of mercury is preferably limited to an amount sufiicient toproduce a high-pressure mercury vapor atmosphere of the order of aboutone or several atmospheres during operation of the lamp but is limitedso as to be all evaporated during operation and at a temperatureslightly lower than the operating temperature of the arc tube, so thatthe mercury vapor atmosphere is unsaturated and the mercury vapor issuperheated during operation of the lamp. The effects of temperature andvoltage fluctuations on the light output and the operatingcharacteristics of the lamp incorporating an arc tube containing suchmeasured amount of mercury are thus minimized.

The electrodes 4, 5 and 6 are connected to the current inlead wires 8, 9and 10, respectively, which are suitably made of molybdenum and have afoliated portion 11 completely embedded in and hermetically fused withthe quartz seals 2 and 3. The starting electrode 6 is constituted oftungsten wire and the outer end thereof overlapsand is welded to thefoliated portion of the inlead 10. The electrodes 4 and 5 arespot-welded to the inner cylindrical ends of the inleads 8 and 9,respectively. The welded joint between the electrode 4 and the currentinlead 8 is indicated at 12 in Fig. 2 of the drawing.

The electrodes 4 and 5 are identical in structure and the followingdescription of the respective parts of the electrode 4, shown in detailin Figs. 2 and 3, also applies to the structure of electrode 5.

v Referring to Figs. 2 and 3 of the drawing, the electrode 4 is made upof a straight center wire 13; the inner wire coil 14, which is slippedover the straight wire; the thorium sliver 15, which is inserted betweenthe coil 14, and the wire 13 from the tip end of the electrode 4, andthe outer fine wire coil 16 slipped over and attached to the inner coil14 so as to be coaxial therewith. These components of the electrode 4are attached to each other by'first welding together the wire 13, thestrip 15 and the coil 14 at the turns of the coil 14 nearest the tip endof the electrode. After these components have been so assembled andattached to each other, the outer coil 16 is slipped over the inner coil14 and the center turns only of the two coils 14 and 16 are weldedtogether as shown at 17.

. The following detailed description of the new electrode is given as anexample of a specific embodiment of the invention useful in the arc tubeof a commercial highpressure mercury vapor lamp incorporating an outerglass jacket, for the arc tube and known in the trade as the 'H400-E1mercury lamp and operating on about 3 amperes and 400 watts.

' Each of the main discharge supporting electrodes for separated about70 millimeters.

such a lamp, in accordance with the present invention, is made up of aninner coil 14 of 30 mil diameter tungsten wire closely wound with32jturns per inch on a straight mandrel 37 mils in diameter. 'The innercoil 14 is 6 millimeters long and is positioned with its inner end about2 millimeters from the tip 18 of the straight wire 13. The diameter ofthe straight tungsten wire 13 is 30 mils and the thorium sliver 15between the coil 14 and the straight wire 13 is 5 mils in thickness,about 5 millimeters long and about 0.5 millimeters wide.

The outer coil 16 is of 5 mil diameter tungsten wire wound on a straightmandrel 88 mils in diameter with 180 turns per inch and is 8 millimeterslong. After coiling the wound wire of coil 16 springs back to give thecoil 16 an outer diameter of about mils. The coil 16 is positioned onthe inner coil 14 with its innermost end extending 0.5 millimeter beyondthe corresponding end of the inner coil 14 and back 1.5 millimeters fromthe tip 18 of the straight wire 13. The innermost end of the coil 16 maybe as much as 2 millimeters from the tip 18 of the wire 13 and flushwith the corresponding end of coil 14. This provides some tolerance inmanufacture.

The electrodes 4 and 5 are mounted in the envelope 1 of the arc tubewith the tips 18 of the straight wires 13 The quartz arc tube is about5% inches in over-all length including the seals 2 and 3, and about 4inch in outer diameter with a Wall thickness of approximately 1millimeter.

I have demonstrated that in lamps having an arc tube so dimensioned andhaving mounted therein a pair of the above-described cooperating maindischarge supporting electrodes 4 and 5, the arc discharge starts andbecomes stable on the average in about 20 cycles of a 60 cyclealternating current supply source after potential is applied to thelamp, whereas similar lamps of identical structure, except for theomission of the outer coil 16 from each of the main electrodes, requireaverage of 120 to cycles of such a source after the application ofpotential for the arc to start and stabilize.

I have demonstrated further that lamps having electrodes provided withthe outer or overwind coil 16 of the present invention have a bettermaintenance of luminous output than prior lamps not equipped with theoverwind electrodes of the present invention.

For example, in actual tests of a group of lamps equipped withelectrodes of the present invention and a group of lamps of identicalstructure except for the omission of the outer coil 16 from each of themain electrodes, it was found that both types of lamps had an initialaverage output of about 51 lumens per watt. At the end of 500 hours, thelamps with the new electrodes had an average output of 50 lumens perwatt; at 2,000 hours, an average output of 46 lumens per Watt, and atthe end of 4,000 hours of operation an output of about 42 lumens perwatt on the average.

In contrast, in the group of lamps equipped with main electrodes fromwhich the overwind coil 16 was omitted, the average output at the end of500 hours of operation was slightly less than 47 lumens per watt; at theend of 2,000 hours, about 41 lumens per watt, and at the end of 4,000hours, about 35 lumens per watt.

It will be understood that I contemplate that changes in the electrodestructure of the present invention may be made without departing fromthe spirit and scope of the invention; for example, where the electrodeis used in lamps of other wattage sizes than that specified in theillustrative example above described, the over-all and componentdimensions of the electrode may be changed to attain the advantages ofthe electrode with lamps of higher or lower current consumption.

Also, the straight wire 13 of the electrode may constitute the currentinlead wire for the electrode and extend through and be hermeticallyunited with the wall of the quartz envelope 1 by one or moreintermediate sealing glasses forming the well-known graded sealconstruction.

Further, While I prefer to extend the outer coil 16 beyond both ends ofthe inner coil 14, as shown in Fig. 2 of the drawing, to provide maximumprotection for the thorium sliver 15 against the bombardment effects ofthe glow discharge during starting, I contemplate that the outer coil 16may terminate at the end of the inner coil 14 in the direction of theseal 2, or short of said end, without affecting the characteristic quickstarting of the arc discharge from the turns of the outer coil 16 andthe subsequent transfer of the arc discharge to the tip 18 of theelectrode with the build-up of the mercury vapor pressure in the arctube.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A refractory metal self-heating electrode for a highpressure gaseousarc discharge device comprising an elongated center wire having an arcdischarge supporting tip, a sliver of thermionically active metal onsaid center wire, a metal wire inner coil fitting snugly around saidcenter Wire and sliver and spaced from said tip, and a metal wire outercoil around and coaxial with said inner coil and engaging the inner coilat a point removed from said tip to receive support therefrom, the turnsof said outer coil being spaced radially from the turns of said innercoil at the ends of said coils next to said tip to minimize heattransfer between the said turns of the respective coils, the said turnsof the outer coil being spaced longitudinally from the said tip and thesaid turns of the inner coil being no closer longitudinally to said tipthan the said turns of the outer coil, the said turns of the outer coilbeing of finer wire in order to have a lower thermal inertia than thoseof the inner coil whereby the said turns of the outer coil are quicklyheated to an arc discharge supporting temperature by a glow dischargeincident at said electrode to facilitate starting of an arc dischargeand minimize electrode sputtering.

2. A refractory metal are discharge supporting selfheating electrodecomprising a straight metal wire having an arc discharge supporting tip,a metal wire inner coil around said straight wire and spacedlongitudinally from said tip, a sliver of thorium metal between saidinner coil and said straight wire and covered by said inner coil, and ametal wire outer coil coaxial to said inner coil extending beyond saidinner coil in both directions and spaced longitudinally from said tip,the center portions of said coils being welded to each other, the saidouter coil being radially spaced from said inner coil except at thewelded-t0- gether center portions of said coils, the end turns of saidouter coil nearest said tip being of finer wire than the Wire of saidinner coil and said straight Wire whereby the said end turns of theouter coil are quickly heated to an arc discharge supporting temperatureby a glow discharge incident at said electrode.

3. A refractory metal self-heating electrode comprising a straight metalwire having an arc discharge supporting tip, a metal wire inner coilfitting around said straight wire and spaced longitudinally from saidtip, a sliver of thorium metal between said inner coil and said straightWire and covered by said inner coil, and a metal wire outer coil aroundand coaxial with said inner coil and engaging the inner coil at a pointremoved from said tip to receive support therefrom, the turns of saidouter coil being spaced radially from the turns of said inner coil atleast at the ends of said coils next to said tip to minimize the heattransfer between the said turns of the respective coils, said outer coilextending beyond said inner coil in the direction of said tip, the saidturns of the outer coil being of finer wire than those of said innercoil and said straight Wire whereby the said end turns of the outer coilare quickly heated to an arc discharge supporting temperature by a glowdischarge incident at said electrode.

References Cited in the file of this patent UNITED STATES PATENTS2,177,703 Francis Oct. 31, 1939 2,241,362 Gustin. et a1. May 6, 19412,667,592 Hilder Jan. 26, 1954 2,687,489 Anderson et al Aug. 24, 1954

